The present invention relates to reducing defects on the surface of a chemical vapor deposition (CVD) silicon dioxide film, and in particular, such a film formed using ozone-TEOS chemistry, and immediately followed by a plasma enhanced chemical vapor deposition (PECVD) silicon dioxide film.
One of the primary steps in the fabrication of modern semiconductor devices is the formation of a thin film on the semiconductor substrate by chemical reaction of gases. Such a deposition process is referred to as chemical vapor deposition or "CVD" for short. Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions take place to produce a desired film. The temperatures at which thermal CVD processes operate can damage device structures having metal layers. Plasma enhanced CVD (frequently referred to as PECVD) processes on the other hand, promote disassociation of the reactant gases by the application of radio frequency (RF) energy to a reaction zone proximate the substrate surface, thereby creating a plasma of highly-reactive ionic species. The high reactivity of the released ionic species reduces the energy required for a chemical reaction to take place, and thus lowers the required temperature for such CVD processes. The relatively low temperature of a PECVD process makes such processes useful, for example, for the formation of insulating layers over deposited metal layers.
A two layer insulating or dielectric film is often used and provides certain advantages over a single layer film. A low pressure, or sub-atmospheric CVD (SACVD) process can form a film which fills gaps well, such as gaps between metal lines or between polysilicon or other areas. One example of an SACVD process used by Applied Materials uses ozone and is typically in a temperature range of 350-500.degree. C. and a pressure of 20-620 torr. A SACVD process has a slow deposition rate, and thus it is desirable to use a "sandwich" layer with a second, PECVD layer deposited on top of the first layer. The PECVD layer can be created much more quickly, reducing the time required to deposit a layer of desired thickness. The second layer can then be planarized using a chemical mechanical polish (CMP).
Upon the termination of a SACVD process, it is possible for defects to form on the film surface due to reactions occurring after the termination of the process. These reactions can affect the integrity of the film, or the interface with any subsequent film.
In particular, where a PECVD process is used to deposit a second silicon dioxide film on top of the film formed by the SACVD process, inter-layer defects can be problematic. In particular, a subsequent etch to form a metal connection through the layer may etch laterally into the weak interface, thus causing metal to be deposited where it is not desired and preventing clean, vertical lines for the metal connection.
Each silicon dioxide layer may contain dopants which produce a particular type of dielectric layer. The layers can be undoped silica glass (USG), or can contain dopants such as phosphorous doped silicon glass (PSG) or boron and phosphorous doped silicon glass (BPSG). The dual layers can be used, for instance, for pre-metal dielectric (PMD) or for inter-metal dielectric (IMD) layers, or for an insulating or passivating layer.
The two layers can be different combinations of PSG, BPSG or USG. In particular, a SACVD USG layer could be followed by a PECVD USG layer. Alternately, a SACVD PSG layer could be followed by a PECVD layer of USG or PSG. A SACVD BPSG layer could be followed by a PECVD layer of BPSG, PSG, or USG. A preferred silicon source is TEOS, while TMB or TEB may be used for the boron source and TEPO, TEP, TMP OR TMOP may be used for the phosphorous source.
One method of creating the two layers is to deposit the first layer, and then move the wafer to a second reactor chamber for depositing the plasma enhanced film. The disadvantage of this method is the need for a second reactor chamber, and the need to move the wafer to the other chamber, requiring additional time.
A second method uses a chamber which has the capability for both SACVD (20-620 torr) and PECVD (0.5-20 torr) processes. The chamber is first used for an SACVD process to deposit the first layer, and then the wafer is removed from the chamber and the chamber is cleaned. The wafer is subsequently brought back into the chamber to deposit the plasma enhanced film. This eliminates the forming of defects due to reactions with materials from the first process in the second process by having the chamber cleaned between processes. This process requires only a single chamber, but also requires the time for removing and replacing the wafer with the intermediate steps of cleaning the chamber.
Simply doing the two depositions back to back in the same chamber without an intermediate clean has been found to produce a weak interface between the two silicon dioxide layers.
FIG. 3 illustrates a cross-sectional view of a dual layer film having a lower layer 54 and an upper layer 56. A channel 58 is shown being etched through the two layers and filled with metallization. As can be seen, due to a poor interface 60, the etching can eat into the sidewalls at positions 62. As can be seen, the sidewalls are thus not clean, which can affect the performance and integrity of a circuit formed on the wafer.
Defects can occur to cause a weak interface when, at the end of the SACVD step, during pump down, ozone reacts with different reactants in the chamber after the dopant gases and TEOS are turned off. On the start of the PECVD process, the dopants may react with oxygen to form B.sub.2 O.sub.3 or P.sub.2 O.sub.5, in the interface, which could result in a weak interface. Boron and phosphorus oxides mixed in with the silicon oxide are desirable, but not if the boron and phosphorus oxides predominate over the silicon oxide.
Interface problems between other dielectric layers have been addressed in different ways in the prior art. For instance, U.S. Pat. No. 5,271,972 discusses forming a PECVD layer first, and then depositing a thermal CVD layer. In order to reduce the surface sensitivity at the interface, the patent discusses doing a stepwise reduction of power at the end of the PECVD process.
In U.S. Pat. No. 5,426,076, another method is discussed for forming a better interface with a PECVD layer being deposited first with a subsequent thermal CVD layer (THCVD). In this patent, an oxygen plasma (O.sub.2) clean step is performed to remove deposits of liquid TEOS formed in the PECVD process. This prevents TEOS from reacting with the ozone in the subsequent THCVD process.
The present invention is addressed to both a SACVD layer process and a dual dielectric layer in which the PECVD process comes second, not first. There is a need for improving the interface between the two layers while allowing the two processes to occur in-situ, or back-to-back, in the same chamber without an intermediate chamber cleaning step.